1. Field of the Invention
The present invention relates to an ignition apparatus for an internal combustion engine, and more particularly to an ignition apparatus which uses an insulated-gate bipolar transistor (called IGBT, hereinafter) as a switching element.
2. Description of the Related Art
Referring at first to FIG. 6, brief description will be made of a typical one of conventional ignition apparatuses for an internal combustion engine using an IGBT as a switching element, hereinbelow.
An output circuit of electronic control unit (ECU) 1 is constructed by PNP transistor 9, NPN transistor 10 and resistor 11. The transistors 9, 10 alternately repeat the switching operation in response to the signals obtained by computer (CPU) 8, whereby the pulse-like voltage as an ignition control signal is produced to ignition device 2.
The ignition device 2 comprises hybrid IC 19 and IGBT 21. The hybrid IC 19 is composed of resistor 16 for detecting a primary current of ignition coil 20, transistor 17 for restricting the primary current of the coil 20 to a set value by its base potential being controlled, and input resistor 18. IGBT 21 repeats the on-off operation to control the current flowing through a primary winding of the ignition coil 20. Further, V.sub.B indicates a battery terminal and reference numeral 14 denotes high voltage diode inserted between a secondary winding of the ignition coil 20 and a spark plug (not shown) for preventing a reverse voltage appearing when the primary current begins to flow.
FIGS. 7a to 7d show waveforms of voltage or current at various parts of the circuit as shown in FIG. 6. Thereamong, FIG. 7a denotes a waveform of an ignition control signal applied to a gate of IGBT 21 (i.e. IGBT gate by voltage), FIG. 7b that of the primary current flowing through the ignition coil 20, FIG. 7c that of a collector voltage of IGBT 21 and FIG. 7d that of a secondary voltage of the ignition coil 20.
When ECU 1 produces the ignition control signal as shown in FIG. 7a to a gate of IGBT 21, the primary current as shown in FIG. 7b begins to flow. When the primary current increases and the voltage drop across the resistor 16 reaches the operating voltage of the transistor 17, the transistor 17 becomes conductive to thereby decrease the gate voltage of IGBT 21. As a result, IGBT 21 is kept in the active state and the collector voltage thereof rises as shown in FIG. 7c, whereby the primary current of the ignition coil 20 is maintained constant. Namely, the primary current is ready to enter into the saturation condition.
In this circuit as shown in FIG. 6, however, the voltage between the collector and the emitter of IGBT 21 jumps at the beginning of the primary current being restricted, i.e., just before the saturation thereof, because of the relationship of the phase delay in the gate control and the gain of IGBT 21. Accordingly, the primary current flowing through the ignition coil 20 also jumps as indicated by B.sub.1 in FIG. 7b, whereby the severe vibration occurs in the voltage between the collector and the emitter as indicated by C.sub.1 in FIG. 7c. It becomes more severe according to an ignition coil used, when the gain of IGBT 21 increases because of the temperature rise.
Since the primary current jumps and then swings as shown by B.sub.1 in FIG. 7b, a voltage is induced in the secondary winding of the ignition coil 20, as shown by D.sub.1 in FIG. 7d. If this voltage D.sub.1 is high enough, an undesirable spark occurs in a spark plug before the regular spark caused by the secondary voltage D.sub.2 at the proper timing.
To improve such a problem as mentioned above, an ignition apparatus as shown in JP-A 6-53795 is proposed. Referring again to FIG. 6, brief explanation about this prior art will be done, hereinbelow. The ignition apparatus of this prior art is provided with a damper resistor between the gate of IGBT 21 and the juncture of the collector of the transistor 17 and the resistor 18. With such a damper resistor, the response of the IGBT is lowered, whereby the current flowing therethrough is restricted stably.
In the case, however, where an IGBT is connected on the side of an ignition coil, which is low in the potential, i.e., on the side opposite to the battery side, the damper resistor can not suppress the jump of the current sufficiently. Further, since the responsibility of the IGBT is lowered, this prior art is not suited for the high speed switching operation. Moreover, there was a problem that the sufficiently high secondary voltage could not be obtained, because the current cutting-off speed is decreased due to the gate capacitance of the IGBT.
Furthermore, there was a problem as follows. That is, if the gain of the IGBT becomes large because of the temperature rise, for example, the jump or vibration in the collector voltage is increased, whereby the primary current also jumps or vibrates and hence the undesirable high voltage is induced in the secondary winding of the ignition coil.